It is well known that incandescent light bulbs are a very energy inefficient light source—about 90% of the electricity they consume is released as heat rather than light. Fluorescent light bulbs are by a factor of about 10 more efficient, but are still less efficient than a solid state semiconductor emitter, such as light emitting diodes, by factor of about 2.
In addition, incandescent light bulbs have a relatively short lifetime, i.e., typically about 750-1000 hours. Fluorescent bulbs have a longer lifetime (e.g., 10,000 to 20,000 hours) than incandescent lights, but they contain mercury, not an environment friendly light source, and they provide less favorable color reproduction. In comparison, light emitting diodes have a much longer lifetime (e.g., 50,000 to 75,000 hours). Furthermore, solid state light emitters are a very clean “green” light source and can achieve very good color reproduction.
Accordingly, for these and other reasons, efforts have been ongoing to develop solid state lighting devices to replace incandescent light bulbs, fluorescent lights and other light-generating devices in a wide variety of applications. In addition, where light emitting diodes (or other solid state light emitters) are already being used, efforts are ongoing to provide improvement with respect to energy efficiency, color rendering index (CRI Ra), luminous efficacy (1 m/W), color temperature, and or duration of service, especially for indoor applications.
A semiconductor light emitting device utilizes a blue light emitting diode having a main emission peak in the blue wavelength range from 400 nm to 490 nm, and a luminescent layer containing an inorganic phosphor that absorbs blue light emitted by the blue LED and produces an exciting light having an emission peak in a visible wavelength range from green to yellow. The mixture of emitted blue light and excited yellow light produces a white light with correlated color temperature (CCT) around 6500 K.
Almost all the known light emitting semiconductor devices place a phosphor layer in the light emitting path. Phosphors may be disposed in the path of the forward emitted light by a semiconductor light emitting diode in several ways. U.S. Pat. No. 6,351,069 describes an III-V nitride LED covered by a layer of a transparent resin in which a wavelength conversion material is mixed. U.S. Pat. No. 6,630,691 describes growth of LED devices on single crystal luminescent substrates. U.S. Pat. No. 6,696,703 describes the use of thin film phosphor layers disposed over LEDs. U.S. Pat. No. 6,576,488 describes forming conformal phosphor layers on LEDs by electro-phoretic deposition. Directly forming a phosphor layer on an LED or mixing into an epoxy resin to encapsulate the LED must handle the temperature rise issue in the LED itself. The intrinsic phosphor conversion efficiency, for some phosphors, drops dramatically as the temperature increases above approximately the 90° C. threshold. Also, phosphor directly-attached to LED will cause more phosphor degradation over heat and time.
Current state-of-the-art phosphor-converted LED (Pc-LED) technology is inefficient due to the backscattering issue. Phosphor particles within a luminescent layer or cured encapsulation layer are randomly oriented and have particles sizes from about 5˜50 microns which is much bigger than the wavelength of visible light. A portion of the primary short wavelength light emitted by the LED passes through the phosphor layer without impinging on the phosphor particles, and another portion of the primary light emitted by the LED chip impinges the phosphor particles, causing the phosphor particles to emit longer wavelength radiation or scatter the primary short wavelength light. The impingement of primary short wavelength light onto a down-conversion phosphor layer may produce radiations with four components: a forward-transferred down-converted radiation transmitted through the phosphor layer; a back-transferred down-converted radiation reflected from the phosphor layer; a back-transferred primary short wavelength light reflected from the phosphor layer; and a forward transferred primary short wavelength light transmitted through the phosphor layer. The combination of the forward-transferred primary short wavelength radiation and the forward-transferred down-converted radiation produces white light. But the back-transferred primary radiation and down-converted radiation will mostly reflect back into the primary LED chip in current state-of-the art Pc-LED which phosphor layer directly applied to LED, causing significantly phosphor conversion back-scattering loss.